This invention relates to photovoltaic devices for producing usable amounts of electrical energy from incident solar radiation and, more particularly, to the assembly of discrete photovoltaic panels into insulated, sealed frame structures.
Extensive research effort is currently being expended to develop mass production capabilities for photovoltaic cells producing electrical energy from incident solar radiation. Two basic technologies are evolving: one dealing with silicon materials, and another dealing with polycrystalline thin films. A major advantage to the thin films is that an effective photovoltaic cell may be formed from a polycrystalline film produced by production techniques adaptable to large volume operations.
A particularly desirable polycrystalline thin film may be formed from cadmium sulfide although suitable films may be formed generally from compounds of elements in Groups II B and VI A of the periodic table. As used herein, the term cadmium sulfide-type cell refers to a photovoltaic cell incorporating any of these compounds.
U.S. Pat. Nos. 3,880,633, 4,086,101, and 4,104,420, all to Jordan and Lampkin, teach processes for forming a large area photovoltaic panel having a glass substrate with a transparent electrically conductive coating of tin oxide, a layer of polycrystalline cadmium sulfide covered by a heterojunction-forming layer of cuprous sulfide, and overlying conductive materials. U.S. Patent application No. 831,544 further shows a series connected array of photovoltaic cells formed from such a photovoltaic panel.
As hereinbelow used, a photovoltaic or solar panel unit refers to an array of cadmium sulfide-type photovoltaic cells on a common substrate. In a preferred embodiment, the photovoltaic cells are formed according to teachings of the above patents and patents applications and connected in a series configuration by exposed layers of metallic conductors. The cells thus formed are in a back-wall configuration, wherein the activating solar irradiation is incident on the glass substrate and must pass through the layer of cadmium sulfide prior to reaching the heterojunction. The resulting photovoltaic panels are fragile and should desirably be provided with a frame to facilitate handling and installation in an overall power producing network.
In the prior art which was concerned with packaging silicon cells, hermetically sealed enclosures were needed to protect the silicon from atmospheric contamination or the silicon was embedded in a surrounding plastic. Early cadmium sulfide-type photovoltaic cells were generally small and were assembled in interconnected arrays much as the monocrystalline silicon cells were assembled.
One producer of photovoltaic panels, SES, Inc., provides a standard panel which is nominally 8 inches square and is hermetically sealed to protect the cell interconnections. A plurality of these panels are formed into a modular array by plugging the hermetically sealed panels into a frame assembly. This frame assembly has been manufactured of metallic materials and requires interconnecting wiring with such corrosion resistant conductors as gold plated contact surfaces.
One of the main obstacles to large scale use of photovoltaic solar energy conversion devices is the large cost per installed watt of presently available devices. A high percentage of the cost is derived from the need to provide hermetically sealed environments and elaborate frames for the photovoltaic cells and/or panels. The need for low fabrication costs is further dramatized when it is realized that a large potential market for low cost photovoltaics is the underdeveloped regions of the globe where central power distribution grids are simply not in existence.
The solar cell modules which are in the prior art are not capable of being installed and maintained in many areas of the world. A suitable photovoltaic module must be capable of remaining unattended for extended periods of time. Further, the photovoltaic assembly should preferably be safe with respect to some tampering by uneducated peoples. The present sophisticated photovoltaic packaging now available is not believed to be capable of meeting the above problems.